Gotta share this. Had another client get caught up in this pop-up alert scam. DO NOT call any phone numbers if you get one of these.
Your Mac has not been infected with a virus or malware. It is simply a locked pop-up window and there are ways to rid yourself of it.

Follow these instructions to rid yourself of the pop up. They have worked for me.

MacKeeper, a utility and security program for Apple computers, celebrated its fifth birthday in April. But its gift to U.S. consumers who bought the application may be a slice of a $2 million class-action settlement.

AV Comparatives, an Austrian company that evaluates antivirus programs, recently tested the latest trial version of MacKeeper at the request of IDG News Service. It was installed on a fresh, fully patched version of OS X Yosemite, Apple’s latest operating system. In theory, the system should have had no problems.

MacKeeper warned in red in several places with exclamation points that the computer’s condition was “serious” due to more than 500 MB of “junk” files.

After fixing 85 files for free, it warned more than 1,500 need cleaning—but only if the full version of the program was purchased. Those that supposedly need cleaning up included language files.

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This is where I'd normally write an impressive summary of my skills and proficiencies.

As I've said before, I am suspicious of "clean up" apps, even if they're legitimate. (MacKeeper appears not to be legitimate.) I've never seen one that didn't need at the very least very close attention to prevent it from deleting necessary files.

If a user is really suffering from lack of drive space, there are many solutions that don't involved deleting files.

"Design is not just what it looks like and feels like. Design is how it works."
Steve Jobs

ScamZapper is not like most other pop-up blockers, as it targets a specific kind of pop-up: the worst kind. Some pop-ups will lock up Safari, claiming that your computer is infected by a virus and you need to call a bogus technical support number for help, and this is exactly what ScamZapper is designed to prevent. You can learn more about this scam in our support article here: A Browser Pop-up has Taken Over Safari. This is not a virus or adware, so anti-virus or adware software won't detect it. Furthermore, even pop-up blockers usually fail to stop these.

It's important to repeat that if you ever see such a popup don't call any phone numbers or go to any web sites and don't download anything.

"Design is not just what it looks like and feels like. Design is how it works."
Steve Jobs